This invention relates to optical systems and, more particularly, to high-power optical systems that include an intermediate focus of the optical beam.
In many optical systems, the optical beam is focused by the optical elements to an intermediate image at a location within the optical system. When the optical beam is a low-power beam, that intermediate focus poses no problem. However, when the optical beam is a high-power beam, the intermediate focus may have such a high optical power density that the air in the vicinity of the intermediate focus is ionized. The ionization of the air through which the optical beam passes distorts the ionizing optical beam, as well as other optical beams that pass through the intermediate focus, in an uncontrolled manner.
The ionization adversely affects the imaging of the optical beam. High-power optical beams are not normally imaged, but in some cases the optical beam includes both a high-power wavelength component that is not imaged, and a low-power wavelength component that is imaged. An example is a laser designator, in which a high-power designator beam at a first wavelength is propagated in one direction along the optical path from a source within the optical system to an external target, and a lower-power imaging beam at a second wavelength is propagated in the opposite direction along the optical path to a sensor within the optical system. If the optical system involves an intermediate focus, the ionization produced by the high-power optical beam at the intermediate focus results in a wavefront distortion that adversely affects the imaging of the low-power optical beam.
To avoid the ionization effect, the intermediate focus may be formed in a vacuum. There is no air to ionize, and the problems discussed above do not arise. However, a vacuum chamber added to an optical system to contain all or a part of the optical system adds weight and complexity to the optical system, may be difficult to maintain in hostile environments to avoid leaks, requires the use of special light-transparent materials in some cases, and may involve a significant transmission loss at the windows of the vacuum system.
There is a need for a better approach to optical systems that avoids such ionization problems. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical system with an intermediate focus of the optical beam in air or other gas, but which avoids ionization of the gas by a high-power beam that is focused at the intermediate focus. No vacuum chamber is utilized around the location of the intermediate focus, thereby avoiding the weight, complexity, maintenance difficulties, materials requirements, and transmission loss associated with the presence of the vacuum chamber. The present approach is operable with a single high-power wavelength component of the optical beam, but is more advantageously used where there is both a high-power wavelength component and an imaged low-power wavelength component that are transmitted along the same optical path through the optical system.
In accordance with the invention, an optical system has a light source of an optical beam, and a wavefront distortion generator that introduces a known wavefront distortion into at least one wavelength component of the optical beam prior to the formation of an intermediate image. A focusing device receives the optical beam, produces the intermediate image of the optical beam, and outputs the optical beam. An example of a focusing device is an three-mirror anastigmat. After the formation of the intermediate image, a wavefront distortion corrector introduces a wavefront distortion correction into each component of the optical beam into which the known wavefront distortion was introduced by the wavefront distortion generator. The wavefront distortion correction is the reverse of the known wavefront distortion introduced into the optical beam by the wavefront distortion generator. The wavefront distortion generator and the wavefront distortion corrector may each be a reflective optical element or a refractive optical element. The wavefront distortion generator and the wavefront distortion corrector may be separate from the focusing device, or at least one of the wavefront distortion generator and the wavefront distortion corrector may be integral with the focusing device.
In one embodiment, the optical beam has exactly one wavelength component, the wavefront distortion generator introduces the known wavefront distortion into the exactly one wavelength component, and the wavefront distortion corrector introduces the wavefront distortion correction into the exactly one wavelength component. In another embodiment, the optical beam has a first wavelength component and a second wavelength component, the wavefront distortion generator introduces the known wavefront distortion into the first wavelength component but not the second wavelength component, and the wavefront distortion corrector introduces the wavefront distortion correction into the first wavelength component but not the second wavelength component. In this second embodiment, the first wavelength component and the second wavelength component may be propagated in the same direction, or they may be propagated in opposite directions through the optical system.
A method of processing an optical beam comprises the steps of supplying the optical beam, thereafter introducing a known wavefront distortion into a least one wavelength component of the optical beam to form a distorted optical beam, thereafter forming an intermediate image of the distorted optical beam, and thereafter introducing a wavefront distortion correction into each wavelength component of the optical beam into which the known wavefront distortion was introduced. The wavefront distortion correction is the reverse of the known wavefront distortion introduced into the optical beam. This method may be applied to an optical beam having a single wavelength component, or to an optical beam having more than one wavelength component, as described earlier.
The present approach avoids the formation of a high-power-density intermediate image by distorting or aberrating the optical beam prior to its reaching the location of the intermediate image, and then correcting the wavefront distortion of the optical beam after it passes the location of the intermediate image. The wavefront distortion generator for the introduction of a controlled, known wavefront distortion may be designed using conventional optical design techniques. The tracing of the distorted optical beam through the location of the intermediate image allows the power density of the distorted optical beam at that location to be determined, and the required wavefront distortion is selected so that the power density is below that which will ionize the gas present at the location of the intermediate image. Because the wavefront distortion is known from the design parameters, the corresponding reverse wavefront distortion correction may also be readily designed into the wavefront distortion corrector. The wavefront distortion generator and the wavefront distortion corrector are configured to distort the optical beam inversely to each other, and they therefore operate equally well on optical beam wavelength components propagated in the same direction or optical beam wavelength components propagated in opposite directions.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.